The present invention concerns a fluid catalytic cracking process wherein (a) residuum and other heavy oils with high metals contents are cracked to produce useful products, (b) contaminant metals on the catalyst are deactivated and (c) sulfur oxides produced during catalyst regeneration are absorbed by the cracking catalyst in sufficient amount to effect a reduction of sulfur oxides in the flue gas.
The catalytic cracking of various heavier mineral hydrocarbons, for instance, petroleum or other mineral oil distillates such as straight run and cracked gas oils; petroleum residues, etc., has been practiced for many years. As is well known, "gas oil" is a broad, general term that covers a variety of stocks. The term includes light gas oil (boiling range 400.degree. to 600.degree. F.), heavy gas oil (boiling range 600.degree. to 800.degree. F.) and vacuum gas oils (boiling range 800.degree. to about 1100.degree. F.). The petroleum residues have a boiling range from about 1100.degree. F. and up. The vacuum gas oils and residuals together represent the atmospheric reduced crude.
A residual stock is in general any petroleum fraction with a higher boiling range than gas oils. Any fraction, regardless of its initial boiling point, which includes the heavy bottoms, such as tars, asphalts, or other undistilled materials can be termed a residual fraction. Accordingly, a residual stock can be the portion of the crude remaining undistilled at about 1050.degree.-1200.degree. F., or it can be made up of a vacuum gas oil fraction plus the portion undistilled at about 1050.degree.-1200.degree. F. For instance, a topped crude may be the entire portion of the crude remaining after the light ends (the portion boiling up to about 400.degree. F.,) have been removed by distillation. Therefore, such a fraction includes the entire gas oil fraction (400.degree. F. to 1050.degree.-1200.degree. F.) and the undistilled portion of the crude petroleum boiling above 1050.degree.-1200.degree. F.
The behavior of a hydrocarbon feedstock in the cracking reactions depends upon various factors including its boiling point, carbon-forming tendencies, content of catalyst contaminating metals, etc. and these characteristics may affect the operation to an extent which makes a given feedstock uneconomical to employ. Although the cracking catalyst employed can be discarded to prevent a accumulation of poisoning metals in the cracking system, this type of operation represents a substantial cost factor. Improvements in the regeneration of catalysts become even more important as the cost of the catalyst rises and thus the effects of low feedstock quality are less burdensome.
Metallic contaminants are found as innate constituents in practically all crude oils. Upon fractionation of the crudes, the metallic contaminants are concentrated in the residua which normally have initial boiling points of about 1000.degree. F. Such residua are conventionally used as heavy fuels, and it has been found that the metal contaminants therein adversely affect the combustion equipment in which the residua are burned. The contaminants not only form ash, which leads to sludging and the formation of deposits upon boiler tubes, combustion chamber walls, the gas turbine blades, but also attack the refractories which are used to line boilers and combustion chambers and severely corrode boiler tubes and other metallic surfaces with which they come into contact at high temperatures.
Efforts of petroleum refiners to employ heavier fractions of crude oil for catalytic cracking have been handicapped due to the heavy coke laydowns experienced in cracking such feedstocks. Coke build-up in catalytic cracking is caused by a number of factors. The presence of high-boiling aromatics and other hydrocarbon coke-formers in the feed contribute to excess coke formation. In high boiling feedstocks these problems are severe since these fractions contain higher proportions than conventional gas-oil feedstocks of coke formers and metal contaminants, which diminish the selectivity of the catalyst. The higher boiling fractions of many crude oils contain substantial portions of metal contaminants, particularly nickel and vanadium components. These metals deposit on the catalyst during the conversion processes so that regeneration of the catalyst to remove coke does not remove these contaminants. This catalyst poisoning modifies the selectivity of a cracking catalyst, causing the catalyst to convert part of the hydrocarbons in the feed to hydrogen and coke rather than the desired light hydrocarbon product. In some commercial operations coke production frequently becomes so severe, due to catalyst poisoning, as well as coke-formers in the feed, that the feed rate or conversion must be reduced to maintain operations with the unit limitations. It is to be understood, therefore, that the problems of catalyst contamination and coke formation prevent full exploitation of heavy feeds.
Contaminant metals in crudes occur naturally. Although traces of most metals have been found in crude oil, the most abundant heavy metals are vanadium, nickel, iron and copper. These metals are catalysts themselves and catalyze dehydrogenation of hydrocarbons and aromatic condensations when deposited upon the cracking catalyst. Any metal poisons in a fluid catalytic cracker feed, even very small concentrations, will deposit almost quantitatively on the cracking catalyst. These deposits can accumulate to very high levels, eventually causing lowered catalyst performance, increased coke deposits and gas make.
A higher level of metals in feeds is a natural result of processing the heavier, more asphaltenic crudes. For instance, the bulk of metals originally present in a crude will eventually become concentrated in residua such as vacuum-tower bottoms. However, gross metals content cannot be used as a measure of contamination since not all deposited metals are equally effective in producing coke and hydrogen. On a weight basis, nickel and copper are the strongest dehydrogenation catalysts, nickel and copper being about four times as strong as vanadium and about six times as strong as iron. (H. R. Grane, et al., Petrol. Refiner, 40, 5, 170) Copper, however, is typically in very low concentration in feedstocks. Iron which is picked up in vessels and lines due to corrosion and erosion is commonly considered as scale or "tramp" metal and has not been considered as a significant catalyst contaminant.
It is well-known that freshly deposited metals are more active as poisons than "older" metals that have been subjected to numerous cycles in the regenerator-reactor circuit. Upon exposure to such repeated cycles of oxidation/reduction, the poisoning effects of metals contaminants are slowly diminished, but there are some claims that those metals on zeolite catalysts lose their effectiveness more slowly than those on amorphous catalysts (Oil Gas J. 70, (20), 112 (1972)).
Sulfur is also typically present in a reduced crude or residual oil. During the cracking process, some of this sulfur is deposited in the coke which is produced by the cracking process. During the conventional regeneration process sulfur oxides are produced during oxidation of the coke to carbon dioxide.
In the residual oil cracking process, the catalyst material is typically withdrawn continuously from the cracking unit and sent to a regenerator where the coke is burned off. High coke yields from cracking residual oils requires removal of a large quantity of excess energy as heat from the regenerator. When the coke is burned in the regenerator, the sulfur content of the coke is converted to sulfur oxides which are emitted in the flue gas and this may necessitate stack gas scrubbing or some other means of control. The contaminant metals remain on the catalyst and continue to catalyze coking-dehydrogenation reactions unless deactivation or removal of these metals takes place. Moreover, although catalytic cracking of residual oils can be more attractive than other processes for utilizing the residual oils, an extremely large economic investment can be required because of the necessity of auxiliary means of removing the excess heat generated by the combustion of the coke in excess of the reactor requirements.
An accompanying problem is the economic investment required for regenerator stack gas scrubbing. When this coke is burned in the regenerator of a catalytic cracker, this sulfur is converted to sulfur oxides. Several cracking catalysts have been developed to reduce sulfur oxide emissions in the flue gas emitted from the fluid catalytic cracking unit, obviating the need for a stack scrubber. In order for these sulfur oxide catalysts to function properly, it is necessary to have an excess of oxygen during the regeneration of the fluid catalytic cracker, more oxygen than is necessary to burn all the coke generated by the cracking process.
In the prior art, it is well-known that the yield of gasoline in the catalytic cracking process decreases with an increase in the coke factor of a catalyst. Duffy and Hart (Chem. Eng. Progr. 48, 344 (1952)) reported that yields of gasoline, based on feed disappearance, dropped when the laboratory-measured coke factor of a catalyst rose from 1.0 to 3.0 in commercial cracking of a feedstock containing highly contaminated stocks. This decreased gasoline yield was matched by an equal increase in gas and coke, the metal contaminants being nickel and vanadium. It has also been theorized that metal contaminants, such as iron, nickel, vanadium and copper markedly alter the character of the cracking reactions. Connor, et al., I.& E.C., 49, No. 2, 281 (1957) teach that the aforesaid metals, when deposited upon the surface of cracking catalysts superimpose their dehydrogenation activity in the cracking reactions and convert into carbonaceous residue and gas some of the material that would ordinarily go into gasoline. Connor indicates an additional explanation to explain the variables affecting the carbon-producing factors of a contaminated catalyst, namely, that the degree of dispersion of the metal over the surface of the catalyst, the higher the carbon-producing factor. Connor indicates these factors are approximately inversely proportional to initial surface area and that the carbon producing factor increases with the proportion of catalyst surface area covered by the contaminant. However, as noted above, in the case of iron particularly, some of the "tramp" metal originating from corrosion and other foreign sources is relatively inert as a contaminant and does not promote dehydrogenation or affect selectivity (H. R. Grane, et al, Petrol. Ref. 40, No. 5 (1961) 170). The detrimental effect of the so-called "tramp metals" and other metals in dissolved or suspended form in the feedstock or originating in corrosion of equipment can be suppressed by use of a reducing gas on a silica-alumina catalyst. (U.S. Pat. No. 2,575,258). When these metals other than as tramp metals exist in organic forms and in low concentrations, their removal can be extremely difficult without adverse effects on other desirable catalyst properties (Oil & Gas. J., p. 75, Dec. 11, 1961). Grane reported, op. cit, that when catalysts containing these metals are exposed to the alternating oxidizing and reducing cycles of the regenerator and of the reactor, the activity of the metal contaminants in coke formation decreased but that an increase in oxygen from 4 to 21 percent or length or temperature of the regeneration cycle had little effect. A repeat program carried out at 1050.degree. F. instead of 900.degree. F. gave almost the same results.
Foster, U.S. Pat. No. 3,122,511, teaches demetallization of a silica-alumina cracking catalyst where the hydrocarbon feed is highly contaminated with nickel, iron and/or vanadium by treating the catalyst with a sulfiding vapor, chlorinating the catalyst, followed by washing with an aqueous medium. Connor, et al., U.S. Pat. No. 3,123,548, teaches removal of metallic impurities from silica-alumina cracking catalyst with use of hydrogen sulfide gas at an elevated temperature, then with molecular oxygen and a suspension of a cation exchange resin in an aqueous medium. Similarly, methods are taught in U.S. Pat. Nos. 3,539,290 (elevated oxidizing temperature and fluid wash); 3,073,675 (an ion-exchange process); 3,162,595 (solvent extraction); French Pat. No. 1,363,355 (an ion-exchange process) (CA, 62, 7563c); Belgian Pat. No. 626,409 (an ion-exchange process) (CA, 60, 9080d); U.S. Pat. No. 3,293,192 (regeneration of zeolite catalysts with steam and/or temperatures of 1300.degree.-1700.degree. F.); U.S. Pat. No. 3,008,896 (regenerating used catalysts from residual oils by a stripping gas or medium); U.S. Pat. No. 3,041,270 (an ion-exchange process).
The primary object of this invention accordingly is to provide a fluid cracking process for proper utilization of cracking catalysts used in processing heavy oils such as residual oil, reduced and whole crudes, gas oil, shale oil, etc. wherein metals deposited on the catalyst are rapidly deactivated concurrently with a reduction of sulfur oxide emissions during the fluid catalytic cracking process.
Another object of this invention is to provide a process wherein sulfur oxide emissions from the fluid catalytic cracking process are reduced by absorption by the catalyst.
Another object of this invention is to provide a process wherein sulfur oxide absorbent cracking catalysts are subjected to an oxidizing atmosphere having an excess of oxygen present.
Another object of this invention is to provide a process for the catalytic cracking of heavy, asphaltenic crudes containing high levels of heavy metals. Another object is to reduce the coke factor of the cracking catalyst and thus increase yields of gasoline from the cracking stock.
These and other objects and advantages of the present invention will become clear from the following specification. These objects have been attained using the process of the present invention.